1. Technical Field
The present invention relates to an optical low pass filter.
2. Related Art
Solid-state image sensors such as a CCD or a CMOS are widely used in video cameras and digital still cameras to capture optical images produced by a receive lens by converting the images into electronic signals. These solid-state image sensors have a structure in which photodiodes are arranged in a regular manner. Here, if the sampling frequency determined from the arranged spacing is surpassed by the spatial frequency of an optical image, moire or other false signals may occur. In order to prevent such false signals, an optical low pass filter using birefringent plates is placed between the receive lens and the solid-state image sensor. The optical low pass filters that are in use include a two-point separation type using a single birefringent plate and a high-performance four-point separation type in which either a retardation plate or a birefringent plate is sandwiched between two birefringent plates.
In order to form such an optical low pass filter, it is necessary to bond solidly together either birefringent plates with one another or a birefringent plate with a retardation plate, without producing bubbles. This bonding process has been performed manually in the past, causing a lot of burden for workers, and sometimes hampered stable supply of good-quality products because differences in the skill of workers created problems, including variability in the thickness of the bond layers, and prevented production of goods with a stable quality.
Under such circumstances, techniques have been proposed to keep a constant thickness for the bond layers by automating the bonding process and applying a proper amount of bond to optical elements (e.g. JP-A-2003-29035). Also proposed are techniques to enhance the bond strength by using bond and high melting wax (e.g. JP-Y-61-28181).
However, the techniques disclosed in JP-A-2003-29035 and JP-Y-61-28181 both use a curable bond and, thus, have been unable to solve such problems as bubbles being mixed into the bond or the appearance being deteriorated through spillage of the bond from bonded ends.